One of the most important processes in the fabrication of semiconductor integrated circuits (ICs) is photolithography. Optical photolithography involves reproducing an image from an optical mask in a layer of photoresist that is supported by underlying layers of a semiconductor substrate assembly. Photolithography is one of the most complicated and critical processes in the fabrication of ICs. The ability to reproduce precise images in a photoresist is crucial to meeting demands for increasing device density.
In the photolithographic process, first, an optical mask is positioned between a radiation source and the photoresist layer on the underlying layers. The radiation source can be, for example, visible light or ultraviolet radiation. Then, the image is reproduced by exposing the photoresist to radiation through the optical mask. Portions of the mask contain an opaque layer, such as, for example, chromium, that prevents exposure of the underlying photoresist. Remaining portions of the mask are transparent, allowing exposure of the underlying photoresist.
The layers underlying the photoresist layer, generally include one or more individual layers that are to be patterned. That is, when a layer is patterned, material from the layer is selectively removed. The ability to pattern layers of material enables ICs to be fabricated. In other words, the patterned layers are used as building blocks in individual devices of the IC. Depending on the type of photoresist utilized (i.e., positive type or negative type), exposed photoresist is either removed when the substrate is contacted with a developer solution, or the exposed photoresist becomes more resistant to dissolution in the developer solution. Thus, a patterned photoresist layer is able to be formed on underlying layers.
One of the problems experienced with conventional optical photolithography is the difficulty of obtaining uniform exposure of photoresist underlying transparent portions of the mask. It is desired that the light intensity exposing the photoresist be uniform to obtain optimum results.
When sufficiently thick layers of photoresist are used, the photoresist must be (or become) partially transparent upon exposure, so that photoresist at the surface of underlying layers is exposed to a substantially similar extent as the photoresist at the outer surface. Often, however, light that penetrates the photoresist is reflected back toward the light source from the surface of the underlying layers of the substrate assembly. The angle at which the light is reflected is dependent on the topography of the surface of the underlying layers and the type of material of the underlying layers. The reflected light intensity can vary in the photoresist throughout its depth or partially though its depth, leading to nonuniform exposure and undesirable exposure of the photoresist. Such exposure of the photoresist can lead to poorly controlled features (e.g., gates, metal lines, etc.) of the IC.
In an attempt to minimize the variable reflection of light in a photoresist layer, antireflective coatings have been utilized between the underlying layers of a substrate assembly and the photoresist layer or between the photoresist layer and the radiation source. Such antireflective coatings minimize photoresist exposure from surface reflections, allowing exposure across a photoresist layer to be controlled more easily from the radiation incident on the photoresist from the radiation source.
Typically, antireflective coatings are organic materials. Organic layers can, however, lead to particle contamination in the integrated circuit (IC) due to the incomplete removal of organic material from the underlying layers after the photolithography step is performed. Such particle contamination can potentially be detrimental to the electrical performance of the IC. Further, the underlying layers upon which the organic materials are formed may be uneven resulting in different thicknesses of the organic material used as the antireflective coating, e.g., thicker regions of the organic material may be present at various locations of the underlying layers. As such, when attempting to remove such organic material, if the etch is stopped when the underlying layers are reached, then some organic material may be left. If the etch is allowed to progress to etch the additional thickness in such regions or locations, then the underlying layers may be undesirably etched (e.g., punchthrough of an underlying layer may occur).
Inorganic antireflective layers have also recently been introduced. For example, silicon-rich silicon dioxide, silicon-rich nitride, and silicon-rich oxynitride have been utilized as inorganic antireflective layers. Such inorganic antireflective layers have been utilized, for example, in the patterning of metal lines and polysilicon gates.
After a patterned photoresist layer is formed on a substrate, many other processes are typically performed in the fabrication of ICs. For example, the photoresist can act as an implantation barrier during an implant step. The photoresist can also be utilized to define the outer perimeter of an area (e.g., a contact hole) that is etched in the substrate or individual layers therein. Once again, the photoresist acts as a barrier during the etching process.
One common photolithographic process involves utilizing the patterned photoresist layer over a pad oxide layer and silicon nitride layer on a supporting substrate. The pad oxide layer is utilized as a stress buffer due to the volumetric increase of adjacent growing oxide and the large difference in thermal expansion coefficients of the silicon wafer and the silicon nitride layer that are problematic during subsequent thermal oxidation. The patterned photoresist layer is utilized to selectively remove the pad oxide layer and silicon nitride layer (e.g., LPCVD silicon nitride) in field regions of a substrate.
After such field regions are defined by the remaining regions of the pad oxide layer and the silicon nitride layer, the patterned photoresist layer is removed. Then, the field regions of the substrate are oxidized, for example, using a wet oxidation process, to form field oxide in the field regions. The silicon nitride layer acts as a barrier to oxygen diffusion, preventing oxidation in underlying active regions. This technique is well known as the LOCOS (Local Oxidation of Silicon) process. One recurring problem, however, with the LOCOS process is encroachment of field oxide under the edges of the silicon nitride in the active regions. This is often referred to as the "bird's beak" phenomenon. As device density increases, the bird's beak problem becomes more problematic because the active region containing the bird's beak is essentially unusable for the fabrication of devices.
Another well known electrical isolation technique is trench isolation. In trench isolation, a trench is etched in the substrate and then filled with deposited oxide. Trench isolation is referred to as shallow trench isolation (STI) or deep trench isolation (DTI), depending on the depth of the trench etched in the substrate. After the oxide is deposited to fill the trench, it is patterned so that the oxide is removed from areas of the substrate outside of the trench etched in the substrate. Conventional photolithography is utilized to pattern the oxide.